U.S. patent application number 12/762601 was filed with the patent office on 2010-08-12 for frequency modulation circuit, transmitter, and communication apparatus.
Invention is credited to Toru MATSUURA.
Application Number | 20100203852 12/762601 |
Document ID | / |
Family ID | 39029807 |
Filed Date | 2010-08-12 |
United States Patent
Application |
20100203852 |
Kind Code |
A1 |
MATSUURA; Toru |
August 12, 2010 |
FREQUENCY MODULATION CIRCUIT, TRANSMITTER, AND COMMUNICATION
APPARATUS
Abstract
A bandpass type delta sigma modulation section 15 performs delta
sigma modulation on an inputted modulation signal such that
quantization noise is reduced in a frequency band which requires
low noise. An LPF 16 removes a noise component in a high frequency
region from the signal on which the delta sigma modulation has been
performed. A frequency modulation circuit 1 reduces noise in the
frequency band which requires low noise with the bandpass type
delta sigma modulation section 15 and the LPF 16, and reduces noise
in the vicinity of a direct current component DC with a feedback
comparison section 11 and a loop filter 12.
Inventors: |
MATSUURA; Toru; (Osaka,
JP) |
Correspondence
Address: |
WENDEROTH, LIND & PONACK L.L.P.
1030 15th Street, N.W., Suite 400 East
Washington
DC
20005-1503
US
|
Family ID: |
39029807 |
Appl. No.: |
12/762601 |
Filed: |
April 19, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11878286 |
Jul 23, 2007 |
|
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12762601 |
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Current U.S.
Class: |
455/102 |
Current CPC
Class: |
H03C 3/0941 20130101;
H03C 3/0975 20130101; H03C 3/095 20130101 |
Class at
Publication: |
455/102 |
International
Class: |
H04B 1/04 20060101
H04B001/04 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 28, 2006 |
JP |
2006-206247 |
Claims
1. A transmission circuit for generating and outputting a
transmission signal based on input data, the transmission circuit
comprising: a signal generation section operable to generate an
amplitude signal and a phase signal based on an amplitude component
and a phase component which are obtained by performing signal
processing on the input data; an amplitude amplifying section
operable to output a signal which is controlled according to the
amplitude signal; a frequency modulation circuit comprising: a
bandpass type delta sigma modulation section operable to perform
delta sigma modulation on the phase signal such that quantization
noise is suppressed in a frequency band which requires low noise; a
loop filter operable to output a signal which is the phase signal a
high-frequency component of which is suppressed; a voltage
controlled oscillator operable to frequency-modulate the phase
signal by controlling an oscillatory frequency according to the
signal outputted from the loop filter and the bandpass type delta
sigma modulation, and output a resultant signal as a frequency
modulation signal; and a feedback comparison section operable to
compare the frequency modulation signal which is fed back from the
voltage controlled oscillator with the phase signal, control a
frequency of the phase signal according to the result of the
comparison, and output the frequency-controlled phase signal to the
loop filter an amplitude modulation section operable to
amplitude-modulate the frequency modulation signal by using the
signal outputted from the amplitude amplifying section, and output
the frequency-modulated and amplitude-modulated signal as a
transmission signal.
2. A communication apparatus comprising: the transmission circuit
according to claim 1 operable to generate a transmission signal;
and an antenna operable to output the transmission signal generated
by the transmission circuit.
3. The communication apparatus according to claim 2, further
comprising: a reception circuit operable to process a reception
signal received from the antenna, and an antenna duplexer operable
to output the transmission signal generated by the transmission
circuit to the antenna, and output the reception signal received
from the antenna to the reception circuit.
Description
[0001] This application is a divisional application of application
Ser. No. 11/878,286, filed Jul. 23, 2007.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a frequency modulation
circuit which is used in a communication apparatuses such as mobile
phones, wireless LAN devices, or the like, a transmission circuit,
and a communication apparatus. More particularly, the present
invention relates to a frequency modulation circuit which reduces
noise present in a desired frequency band and operates with low
distortion and high efficiency, a transmission circuit using the
frequency modulation circuit, and a communication apparatus.
[0004] 2. Description of the Background Art
[0005] Communication apparatuses such as mobile phones, wireless
LAN devices, or the like need to operate with low power consumption
while maintaining the accuracy of an output signal. Such
communication apparatuses require a frequency modulation circuit
which outputs a frequency modulation signal having low distortion
and operates with high efficiency and a transmission circuit using
the frequency modulation circuit. The following will describe a
conventional frequency modulation circuit.
[0006] As an example of a conventional frequency modulation
circuit, there exists a frequency modulation circuit which is
disclosed in Wendell B. Sander, et al, "Polar Modulator for
Multi-mode Cell Phones", US, Tropian, Inc. FIG. 13 is a block
diagram showing an exemplarily configuration of a conventional
frequency modulation circuit 500. As shown in FIG. 13, the
conventional frequency modulation circuit 500 includes first and
second arithmetic units 507 and, 508, a loop filter 502, a voltage
controlled oscillator (VCO) 503, a frequency digital converter
(FDC) 504, a DA converter (DAC) 505, and a low pass filter (LPF)
506.
[0007] A modulation signal is inputted to the conventional
frequency modulation circuit 500 through an input terminal. The
modulation signal is inputted to the VCO 503 through the first
arithmetic unit 507, the second arithmetic unit 508 and the loop
filter 502. The modulation signal is also inputted to the VCO 503
through the DAC 505 and the LPF 506. The VCO 503
frequency-modulates the modulation signal by controlling an
oscillatory frequency according to the inputted modulation signal,
and outputs a resultant signal as a frequency modulation
signal.
[0008] The second arithmetic unit 508 adds or subtracts a constant
to or from the inputted modulation signal to control the center
frequency of the modulation signal. FDC 504 converts the frequency
of the frequency modulation signal outputted by the VCO 503 into a
digital value, and outputs the converted digital value to the first
arithmetic unit 507. The first arithmetic unit 507 adds the digital
value outputted by the FDC 504 to the modulation signal outputted
by the second arithmetic unit 508, and outputs a resultant signal
to the loop filter 502. The loop filter 502 suppresses the
high-frequency component of the modulation signal outputted by the
first arithmetic unit 507. In other words, the FDC 504, the first
arithmetic unit 507, and the loop filter 502 constitute a feedback
loop which stabilizes the frequency of the frequency modulation
signal outputted by the VCO 503.
[0009] Meanwhile, the DAC 505 converts the inputted modulation
signal into an analog signal. The LPF 506 serves to suppress noise
such as quantization noise which is generated with the processing
by the DAC 505.
[0010] FIG. 14 illustrates a problem of the conventional frequency
modulation circuit 500. FIG. 14 shows a relationship between noise
and a frequency which are included in each of an output signal Ax
of the DAC 505, an output signal Bx of the LPF 506 when the LPF 506
is not connected to an input of the VCO 503, and an input signal Cx
to VCO 503 when the LPF 506 is connected to the input of the VCO
503. It is noted that the symbols Ax to Cx in FIG. 14 correspond to
the points Ax to Cx in FIG. 13, respectively.
[0011] As shown in FIG. 14, the output signal Ax of the DAC 505
includes substantially uniform quantization noise and the like. The
LPF 506 removes noise of a high frequency region from the output
signal Ax of the DAC 505. Since the LPF 506 is connected to the
input of the VCO 503, the output signal Bx of the LPF 506 is
inputted as the input signal Cx to the VCO 503 after noise in the
vicinity of a direct current component DC is removed by the
operation of the feedback loop of the conventional frequency
modulation circuit 500.
[0012] As shown in FIG. 14, however, the noise of the input signal
Cx to the VCO 503 is not always sufficiently reduced in a frequency
band which requires low noise (the shaded area in FIG. 14).
Therefore, the conventional frequency modulation circuit 500 has a
problem that it cannot output a frequency modulation signal the
noise of which is sufficiently reduced in the frequency band which
requires low noise.
[0013] There is a possibility that in the case where the
conventional frequency modulation circuit 500 is installed in a
communication apparatus which performs transmission and reception
concurrently, the noise generated at the conventional frequency
modulation circuit 500 overlaps with a receiving band of the
communication apparatus, and this adversely affects the receiving
quality of the communication apparatus.
SUMMARY OF THE INVENTION
[0014] Therefore, an object of the present invention is to provide
a frequency modulation circuit which reduces the noise present in
the frequency band requiring low noise and operates with low
distortion and high efficiency, a transmission circuit using the
frequency modulation circuit, and a communication apparatus.
[0015] The object of the present invention is directed to a
frequency modulation circuit which frequency-modulates and outputs
an input signal. In order to achieve the above-mentioned object,
the frequency modulation circuit of the present invention comprises
a bandpass type delta sigma modulation section operable to perform
delta sigma modulation on the input signal such that quantization
noise is suppressed in a frequency band which requires low noise; a
loop filter operable to output a signal which is the input signal a
high-frequency component of which is suppressed; a voltage
controlled oscillator operable to frequency-modulate the input
signal by controlling a oscillatory frequency according to the
signal outputted from the loop filter and the bandpass type delta
sigma modulation, and output a resultant signal as a frequency
modulation signal; and a feedback comparison section operable to
compare the frequency modulation signal which is fed back from the
voltage controlled oscillator with the input signal, control a
frequency of the input signal according to the result of the
comparison, and output the frequency-controlled input signal to the
loop filter.
[0016] It is noted that the frequency modulation circuit may
comprise a compensating filter operable to filter and output the
input signal to the bandpass type delta sigma modulation section to
compensate output characteristics of the bandpass type delta sigma
modulation section.
[0017] Preferably, the feedback comparison section includes a
frequency digital converter operable to convert a frequency of the
frequency modulation signal outputted by the voltage controlled
oscillator into a digital value based on a predetermined manner;
and an arithmetic section operable to add or subtract the digital
value converted by the frequency digital converter to or from the
input signal.
[0018] Preferably, the frequency modulation circuit may comprise a
lowpass type delta sigma modulation section operable to perform
delta sigma modulation with signal transfer characteristics of
lowpass type on the signal outputted from the loop filter, and
output to the voltage controlled oscillator the signal on which the
delta sigma modulation has been performed. In this case, the
frequency modulation circuit may further comprise a second
arithmetic section operable to input to the arithmetic section a
signal a center frequency of which is controlled by adding or
subtracting a constant to or from the input signal.
[0019] The frequency modulation circuit may further comprise a
lowpass type delta sigma modulation section operable to perform
delta sigma modulation with signal transfer characteristics of
lowpass type on the input signal, and output to the feedback
comparison section the signal on which the delta sigma modulation
has been performed, and the feedback comparison section may include
a frequency divider operable to frequency-divide the frequency
modulation signal outputted by the voltage controlled oscillator
with the signal on which the delta sigma modulation has been
performed by the lowpass type delta sigma modulation section; and a
comparison section operable to compare a predetermined reference
signal with the signal which has been frequency-divided by the
frequency divider, and control the oscillatory frequency of the
voltage controlled oscillator such that both of the signals are
synchronized.
[0020] The present invention is directed to a transmission circuit
including the above-mentioned frequency modulation circuit. The
transmission circuit comprises a signal generation section operable
to generate an amplitude signal and a phase signal based on an
amplitude component and a phase component which are obtained by
performing signal processing on input data; an amplitude amplifying
section operable to output a signal which is controlled according
to the amplitude signal; the frequency modulation circuit operable
to frequency-modulate and output the phase signal as a frequency
modulation signal, and an amplitude modulation section operable to
amplitude-modulate the frequency modulation signal by using the
signal outputted from the amplitude amplifying section, and output
the frequency-modulated and amplitude-modulated signal as a
transmission signal.
[0021] Further, the present invention is directed to a
communication apparatus including the above-mentioned transmission
circuit. The communication apparatus comprises the transmission
circuit operable to generate a transmission signal; and an antenna
operable to output the transmission signal generated by the
transmission circuit. The communication apparatus may comprise a
reception circuit operable to process a reception signal received
from the antenna, and an antenna duplexer operable to output the
transmission signal generated by the transmission circuit to the
antenna, and output the reception signal received from the antenna
to the reception circuit.
[0022] According to the above-mentioned present invention, noise is
reduced in the frequency band which requires low noise by the
bandpass type delta sigma modulation section, and noise in the
vicinity of a direct current component DC are further reduced by
the feedback loop. Thus, a frequency modulation signal the noise of
which is reduced can be outputted in the frequency band which
requires low noise.
[0023] These and other objects, features, aspects and advantages of
the present invention will become more apparent from the following
detailed description of the present invention when taken in
conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] FIG. 1 is a diagram showing an exemplary configuration of a
frequency modulation circuit 1 according to a first embodiment of
the present invention;
[0025] FIG. 2 illustrates characteristics of a bandpass type delta
sigma modulation section 15;
[0026] FIG. 3 illustrates the operation of the frequency modulation
circuit 1 according to the first embodiment of the present
invention;
[0027] FIG. 4 is a diagram showing a detailed configuration of the
bandpass type delta sigma modulation section 15;
[0028] FIG. 5 is a diagram showing an exemplary configuration of a
frequency modulation circuit 2 according to a second embodiment of
the present invention;
[0029] FIG. 6A illustrates an example of output characteristics of
each section of the frequency modulation circuit 2 when the
function of a compensating filter 21 is OFF;
[0030] FIG. 6B illustrates an example of output characteristics of
each section of the frequency modulation circuit 2 when the
function of the compensating filter 21 is ON;
[0031] FIG. 7 is a diagram showing an exemplary configuration of a
frequency modulation circuit 3a according to a third embodiment of
the present invention;
[0032] FIG. 8 illustrates characteristics of a lowpass type delta
sigma modulation section 35;
[0033] FIG. 9 is a diagram showing another exemplary configuration
of a frequency modulation circuit 3b according to the third
embodiment of the present invention;
[0034] FIG. 10 is a diagram showing an exemplary configuration of a
frequency modulation circuit 4 according to a fourth embodiment of
the present invention;
[0035] FIG. 11 is a diagram showing an exemplary configuration of a
transmission circuit 5 according to a fifth embodiment of the
present invention;
[0036] FIG. 12 is a diagram showing an exemplary configuration of a
communication apparatus 200 according to a sixth embodiment of the
present invention;
[0037] FIG. 13 is a diagram showing an exemplary configuration of a
conventional frequency modulation circuit 500; and
[0038] FIG. 14 illustrates a problem of the frequency modulation
circuit 500.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
First Embodiment
[0039] FIG. 1 is a block diagram showing an exemplary configuration
of a frequency modulation circuit 1 according to a first embodiment
of the present invention. As shown in FIG. 1, the frequency
modulation circuit 1 according to the first embodiment comprises a
feedback comparison section 11, a loop filter 12, a voltage
controlled oscillator (VCO) 13, a bandpass type delta sigma
modulation section 15, and a low pass filter (LPF) 16.
[0040] A modulation signal is inputted to the frequency modulation
circuit 1 through an input terminal. The modulation signal is
inputted to the VCO 13 through the feedback comparison section 11
and the loop filter 12. The modulation signal is also inputted to
the VCO 13 through the bandpass type delta sigma modulation section
15 and LPF 16. The VCO 13 frequency-modulates the modulation signal
by controlling an oscillatory frequency based on the inputted
modulation signal, and outputs a resultant signal as a frequency
modulation signal.
[0041] The feedback comparison section 11 returns and inputs the
frequency modulation signal outputted by the VCO 13. The feedback
comparison section 11 compares the frequency of the inputted
modulation signal with the frequency of the returned frequency
modulation signal, and outputs to the loop filter 12 the modulation
signal the frequency of which is controlled according to the result
of the comparison. The loop filter 12 suppresses the high-frequency
component of the controlled modulation signal outputted from the
feedback comparison section 11. In other words, the feedback
comparison section 11, the loop filter 12, and the VCO 13
constitute a feedback loop which stabilizes the frequency of the
frequency modulation signal outputted by the VCO 13.
[0042] The bandpass type delta sigma modulation section 15 performs
delta sigma modulation on the inputted modulation signal, and
outputs a resultant signal as a delta sigma modulation signal. A
detailed configuration of the bandpass type delta sigma modulation
section 15 is shown in FIG. 4. In the case of the configuration in
FIG. 4, noise transfer function is "1+2Z.sup.-2+Z.sup.-4" having a
zero point at .pi./2 and low noise at a frequency of fs/4. The LPF
16 suppresses noise such as quantization noise generated with the
processing by the bandpass type delta sigma modulation section
15.
[0043] FIG. 2 shows characteristics of the bandpass type delta
sigma modulation section 15 in FIG. 1. As shown in FIG. 2, the
bandpass type delta sigma modulation section 15 has signal transfer
characteristics of bandpass type, and has noise characteristics
reverse to the signal transfer characteristics due to the effect of
noise shaping. Thus, the characteristics of the bandpass type delta
sigma modulation section 15 is designed such that quantization
noise is minimized in a frequency band (the shaded area in FIG. 2)
which requires low noise by the delta sigma modulation.
[0044] FIG. 3 illustrates the operation of the frequency modulation
circuit 1 according to the first embodiment of the present
invention. FIG. 3 shows a relationship between noise and a
frequency which are included in each of an output signal A of the
bandpass type delta sigma modulation section 15, an imaginary
output signal B of the LPF 16 when the output of the LPF 16 is not
connected to the input of the VCO 13 (hereinafter referred to
merely as the output signal B of the LPF 16), an input signal C to
the VCO 13 when the output of the LPF 16 is connected to the input
of the VCO 13 (hereinafter referred to merely as the output signal
C to the VCO 13). It is noted that the symbols A to C in FIG. 3
correspond to the points A to C in FIG. 1, respectively.
[0045] The modulation signal inputted through the input terminal
becomes the output signal A the quantization noise of which is
minimized in a frequency band (the shaded area in FIG. 3) which
requires low noise by the optimally-designed bandpass type delta
sigma modulation section 15. The output signal A becomes the output
signal B such that the noise of the output signal A in a high
frequency region including at least the frequency band which
requires low noise is removed by the LPF 16. The output signal B
becomes the input signal C the noise of which in the vicinity of a
direct current component DC is reduced by the operation of the
feedback loop of the frequency modulation circuit 1, and is
outputted to the VCO 13.
[0046] Therefore, the VCO 13 changes the oscillatory frequency
according to the input signal C and outputs a frequency modulation
signal noise of which is reduced in the frequency band which
requires low noise. As seen from FIG. 3, since the conventional
frequency modulation circuit 500 without the bandpass type delta
sigma modulation section 15 cannot sufficiently reduce quantization
noise in the frequency band which requires low noise, noise
characteristics (the dotted line in FIG. 3) of the input signal Cx
inputted to the VCO 13 is extremely inadequate in comparison to the
present invention.
[0047] As mentioned above, according to the frequency modulation
circuit 1 of the first embodiment of the present invention, the
noise is reduced in the frequency band which requires low noise by
the bandpass type delta sigma modulation section 15, and the noise
in the vicinity of the direct current component DC is reduced by
the feedback loop. Thus, the frequency modulation circuit 1 can
output the frequency modulation signal the noise of which is
reduced in the frequency band which requires low noise. When
installed in a communication apparatus which performs transmission
and reception concurrently, the frequency modulation circuit 1
reduces noise present in a band which overlaps with a receiving
band of the communication apparatus, thereby preventing noise
generated at the frequency modulation circuit 1 from adversely
affecting the receiving quality of the communication apparatus.
Second Embodiment
[0048] Although it is ideal to design the bandpass type delta sigma
modulation section 15 so as to obtain desired output
characteristics as described in the above first embodiment, this
design is practically difficult.
[0049] A second embodiment will describe a frequency modulation
circuit which even though the bandpass type delta sigma modulation
section 15 is not designed so as to obtain a desired output
characteristics, makes total characteristics close to a desired
value by compensating the output characteristics.
[0050] FIG. 5 is a block diagram showing an exemplary configuration
of a frequency modulation circuit 2 according to the second
embodiment of the present invention. As shown in FIG. 5, the
frequency modulation circuit 2 according to the second embodiment
comprises a feedback comparison section 11, a loop filter 12, a VCO
13, a compensating filter 21, a bandpass type delta sigma
modulation section 15, and a LPF 16. The frequency modulation
circuit 2 differs from the frequency modulation circuit 1 according
to the above-mentioned first embodiment in that the compensating
filter 21 is provided in the previous stage of the bandpass type
delta sigma modulation section 15. The compensating filter 21
serves to compensate the output characteristics of the bandpass
type delta sigma modulation section 15. It is noted that the same
components as those in the above-mentioned first embodiment are
designated by the same reference numerals, and the description
thereof will be omitted.
[0051] The operation of the compensating filter 21 will be
described using FIGS. 6A and 6B. FIG. 6A illustrates an example of
output characteristics of each section of the frequency modulation
circuit 2 when the function of the compensating filter 21 is OFF.
FIG. 6B illustrates an example of output characteristics of each
section of the frequency modulation circuit 2 when the function of
the compensating filter 21 is ON. It is noted that FIGS. 6A and 6B
show a relationship between power and a frequency which are
included in each of an output signal E of the compensating filter
21, an output signal A of the bandpass type delta sigma modulation
section 15, an imaginary output signal B of the LPF 16 when the
output of the LPF 16 is not connected to the input of the VCO 13,
an input signal C to VCO 13 when the output of the LPF 16 is
connected to the input of the VCO 13.
[0052] Since the bandpass type delta sigma modulation section 15 is
not designed so as to obtain a desired output characteristics, the
bandpass type delta sigma modulation section 15 has characteristics
which cannot cover a low frequency side like the output signals A
indicated by the long dashed double-short dashed lines in FIGS. 6A
and 6B. Meanwhile, the characteristics caused by the feedback loop
cannot cover a high frequency side like output signals D indicated
by the long dashed short dashed lines in FIGS. 6A and 6B. There is
a frequency region where the characteristics of the output signals
A and D do not overlap with each other and which cannot be covered
by the output signals A and D. Therefore, the signal power of the
input signal C inputted to the VCO 13 has characteristics in which
a low frequency side is attenuated as indicated by the heavy line
in FIG. 6A.
[0053] The compensating filter 21 filters the inputted modulation
signal such that the characteristics on the low frequency band
side, which cannot be covered by the bandpass type delta sigma
modulation section 15, is raised in advance in order to compensate
the output characteristics of the bandpass type delta sigma
modulation section 15 (the output signal E in FIG. 6B). The signal
power of the input signal C inputted to the VCO 13 can obtain
characteristics in which the low frequency band side is even as
shown in FIG. 6B by the operation of the compensating filter 21
even when the bandpass type delta sigma modulation section 15 is
not designed so as to obtain a desired output characteristics.
[0054] As mentioned above, according to the frequency modulation
circuit 2 of the second embodiment of the present invention, even
when the bandpass type delta sigma modulation section 15 is not
designed so as to obtain desired output characteristics, the
compensating filter 21 compensate the output characteristics of the
bandpass type delta sigma modulation section 15. Thus, the
frequency modulation circuit 2 can output a frequency modulation
signal the noise of which is reduced in the frequency band which
requires low noise.
Third Embodiment
[0055] FIG. 7 is a block diagram showing an exemplary configuration
of a frequency modulation circuit 3a according to a third
embodiment of the present invention. As shown in FIG. 7, the
frequency modulation circuit 3a according to the third embodiment
comprises a second arithmetic unit 32, a feedback comparison
section 11, a loop filter 12, a lowpass type delta sigma modulation
section 35, a VCO 13, a bandpass type delta sigma modulation
section 15, an LPF 16 and an LPF 36. The feedback comparison
section 11 includes a first arithmetic unit 31 and a frequency
digital converter (FDC) 34. It is noted that the same components as
those in the above-mentioned first embodiment are designated by the
same reference numerals, and the description thereof will be
omitted.
[0056] A modulation signal (.DELTA..theta./.DELTA.t, .theta. is a
phase modulation signal) is inputted to the second arithmetic unit
32 through an input terminal. The second arithmetic unit 32 adds or
subtracts a constant to or from the inputted modulation signal to
control the center frequency of the modulation signal. The lowpass
type delta sigma modulation section 35 performs delta sigma
modulation on the modulation signal outputted by the loop filter
12, and outputs to the VCO 13 the modulation signal on which the
delta sigma modulation has been performed. The lowpass type delta
sigma modulation section 35 has signal transfer characteristics of
lowpass type, and reduces noise in the vicinity of a direct current
component DC of the modulation signal by performing the delta sigma
modulation on the modulation signal. FIG. 8 illustrates
characteristics of the lowpass type delta sigma modulation section
35. The LPF 36 suppresses noise such as quantization noise
generated with the processing by the lowpass type delta sigma
modulation section 35.
[0057] The FDC 34 converts the frequency of the frequency
modulation signal outputted by the VCO 13 into a digital value
according to a predetermined manner, and outputs the converted
digital value to the first arithmetic unit 31. The first arithmetic
unit 31 adds or subtracts the digital value outputted by the FDC 34
to or from the modulation signal inputted through the second
arithmetic unit 32, and outputs a resultant signal to the loop
filter 12. In other words, the FDC 34, the first arithmetic unit
31, the loop filter 12, and the VCO 13 constitute a feedback loop
which stabilizes the frequency of the frequency modulation signal
outputted by the VCO 13.
[0058] As mentioned above, according to the frequency modulation
circuit 3a of the third embodiment of the present invention, the
center frequency of the frequency modulation signal outputted by
the VCO 13 is changed by controlling the center frequency of the
modulation signal by the second arithmetic unit 32. The provision
of the lowpass type delta sigma modulation section 35 in the
previous stage of the VCO 13 can more effectively reduce the noise
in the vicinity of the direct current component DC of the
modulation signal. Thus, the frequency modulation circuit 3a can
output a frequency modulation signal the noise of which is reduced
in the frequency band which requires low noise.
[0059] It is noted that in the case where frequency characteristics
required for the LPF 16 corresponds to or approximates that for the
LPF 36, the LPF 16 and the LPF 36 may be integrated as an LPF 37
like the configuration of a frequency modulation circuit 3b as
shown in FIG. 9. Sharing the LPF can reduce a circuit size.
Fourth Embodiment
[0060] FIG. 10 is a block diagram showing an exemplary
configuration of a frequency modulation circuit 4 according to a
fourth embodiment of the present invention. As shown in FIG. 10,
the frequency modulation circuit 4 according to the fourth
embodiment comprises a lowpass type delta sigma modulation section
45, a feedback comparison section 11, a loop filter 12, a VCO 13, a
bandpass type delta sigma modulation section 15, and an LPF 16. The
feedback comparison section 11 includes a frequency divider 41 and
a comparison section 42. It is noted that the same components as
those in the above-mentioned first embodiment are designated by the
same reference numerals, and the description thereof will be
omitted.
[0061] A modulation signal (.DELTA..theta./.DELTA.t, .theta. is a
phase modulation signal) and a reference signal are inputted to the
frequency modulation circuit 4 through two input terminals,
respectively. The modulation signal is inputted to the lowpass type
delta sigma modulation section 45 and the bandpass type delta sigma
modulation section 15. The lowpass type delta sigma modulation
section 45 performs delta sigma modulation with signal transfer
characteristics of lowpass type (cf. FIG. 8) on the inputted
modulation signal, and outputs to the frequency divider 41 the
modulation signal on which the delta sigma modulation has been
performed. The frequency divider 41 frequency-divides the frequency
modulation signal outputted by the VCO 13 with the modulation
signal outputted from the lowpass type delta sigma modulation
section 45. The comparison section 42 compares the frequency of the
inputted reference signal with the frequency of the frequency
modulation signal inputted through the frequency divider 41, and
controls the oscillatory frequency of the VCO 13 such that both of
the signals are synchronized. In other words, the frequency divider
41, the comparison section 42, and the loop filter 12 constitute a
feedback loop.
[0062] As mentioned above, according to the frequency modulation
circuit 4 of the fourth embodiment of the present invention, the
center frequency of the frequency modulation signal outputted by
the VCO 13 can be changed by controlling the oscillatory frequency
of the VCO 13 using the feedback loop. Thus, the frequency
modulation circuit 4 can output a frequency modulation signal the
noise of which is reduced in the frequency band which requires low
noise.
[0063] The first to fourth embodiments have described the frequency
modulation circuits 1 to 4 including the LPF 16 and/or the LPF 36.
However, when quantization noise generated at the bandpass type
delta sigma modulation section 15 and/or the lowpass type delta
sigma modulation sections 35 and 45 are sufficiently reduced, or in
the case of a system which has no problem even when noise is large
in a frequency band which is distant from the frequency band which
requires low noise, the LPF 16 and/or the LPF 36 can be
omitted.
[0064] Naturally, the compensating filter 21 mentioned in the above
second embodiment can be added in the previous stage of the
bandpass type delta sigma modulation section 15 in the frequency
modulation circuits 3a, 3b, and 4 of the third and fourth
embodiments.
Fifth Embodiment
[0065] FIG. 11 is a block diagram showing an exemplary
configuration of a transmission circuit 5 according to a fifth
embodiment of the present invention. As shown in FIG. 11, the
transmission circuit 5 according to the fifth embodiment comprises
a signal generation section 51, a frequency modulation circuit 52,
a regulator 54, and an amplitude modulation section 55. Any one of
the frequency modulation circuits 1 to 4 mentioned in the first to
fourth embodiments is used as the frequency modulation circuit
52.
[0066] The signal generation section 51 generates an amplitude
signal and a phase signal from an input signal. The amplitude
signal is inputted to the regulator 54. Direct-current voltage is
supplied from a power supply terminal 53 to the regulator 54. The
regulator 54 supplies to the amplitude modulation section 55 a
signal which is controlled according to the inputted amplitude
signal. The phase signal is inputted to the frequency modulation
circuit 52. The frequency modulation circuit 52 frequency-modulates
the inputted phase signal, and outputs a frequency modulation
signal. The frequency modulation signal is inputted to the
amplitude modulation section 55. The amplitude modulation section
55 amplitude-modulates the frequency modulation signal with the
signal supplied from the regulator 54, and outputs a resultant
signal as a frequency-modulated and amplitude-modulated modulation
signal. The modulation signal is outputted as a transmission signal
through an output terminal 56.
[0067] Such transmission circuit 5 is referred to as a polar
modulation circuit.
[0068] Such transmission circuit 5 operates with low distortion and
high efficiency and can operate with low noise since the frequency
modulation circuit 52 outputs a frequency modulation signal the
noise of which is reduced in the frequency band which requires low
noise. Thus, when installed in a communication apparatus which
performs transmission and reception concurrently, the transmission
circuit 5 reduces noise present in a band which overlaps with a
receiving band of the communication apparatus, thereby preventing
noise generated at the frequency modulation circuit 52 from
adversely affecting the receiving quality of the communication
apparatus.
Sixth Embodiment
[0069] FIG. 12 is a block diagram showing an exemplary
configuration of a communication apparatus 200 according to a sixth
embodiment of the present invention. Referring to FIG. 12, the
communication apparatus 200 according to the sixth comprises a
transmission circuit 210, a reception circuit 220, an antenna
duplexer 230, and an antenna 240. The transmission circuit 5
mentioned in the above fifth embodiment is used as the transmission
circuit 210.
[0070] The antenna duplexer 230 transmits to the antenna 240 a
transmission signal outputted from the transmission circuit 210,
and prevents the transmission signal from leaking to the reception
circuit 220. The antenna duplexer 230 transmits to the reception
circuit 220 a reception signal inputted from the antenna 240, and
prevents the reception signal from leaking to the transmission
circuit 210. Accordingly, the transmission signal is outputted from
the transmission circuit 210, and released from the antenna 240 to
the exterior space via the antenna duplexer 230. The reception
signal is received by the antenna 240 and then received by the
reception circuit 220 via the antenna duplexer 230.
[0071] The communication apparatus 200 according to the sixth
embodiment uses the transmission circuit 5 according to the fifth
embodiment, thereby ensuring the linearity of the transmission
signal and achieving low distortion of a radio device. Since there
is no branch, such as a directional coupler, on the output of the
transmission circuit 210, loss from the transmission circuit 210 to
antenna 240 can be reduced. Thus, power consumption is reduced at
the time of transmission, with the result that the communication
apparatus 200 is capable of operating as a radio communication
device for a long period of time. Further, the transmission circuit
210 of the communication apparatus 200 reduces noise present in a
band which overlaps with a receiving band of the reception circuit
220, thereby preventing noise generated at the transmission circuit
210 from adversely affecting the receiving quality of the reception
circuit 220. It is noted that the communication apparatus 200 may
comprise only the transmission circuit 210 and the antenna 240.
[0072] While the invention has been described in detail, the
foregoing description is in all aspects illustrative and not
restrictive. It is understood that numerous other modifications and
variations can be devised without departing from the scope of the
invention.
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